This invention relates to materials for storage of hydrogen and, more particularly, to materials for storage of hydrogen which can be produced from rare earth metal alloys.
Materials for storage of hydrogen are in general required to have in addition to the following physical properties:
(1) a ready activation,
(2) a large quantity of hydrogen storage,
(3) small hysteresis, and
(4) flatness of a plateau, a good probability of being produced from plentiful, inexpensive materials.
In the past, materials such as titanium-iron alloys, magnesium alloys, and lanthanum pentanickel or Mm-nickel alloys (Mm designates mischmetals) have been proposed for storage of hydrogen. Among these are particularly noted the rare earth metal alloys.
The lanthanum pentanickel is advantageous since it exhibits a large amount of hydrogen that is absorbed and desorbed and relatively fast hydrogen absorption and release properties. On the other hand, the lanthanum pentanickel suffers from a drawback in that the lanthanum of the material is expensive and it is difficult to employ it in a practical use in economy.
The Mm-nickel alloys have been developed to improve the economy of the lanthanum of the lanthanum pentanickel. The Mm generally contains 40 to 50% by weight of cerium, 20 to 30% by weight of lanthanum, and other various metals such as neodymium, praseodymium, and samarium. The mischmetal pentanickel is activated at room temperature, but necessitate a high pressure hydrogen of 100 kg/cm.sup.2 or higher and has a difficulty of practically employing due to large hysteresis.
In order to overcome this difficulty, a hydrogen storage material which is produced by mixing Mm-nickel alloys with a third element such as aluminum, further mixing the resulting mixture with fourth other metal to decrease the absorption and release pressures at a room temperature and to reduce the hysteresis has been proposed. However, the addition of the third and fourth elements causes another disadvantage of a considerably small amount of hydrogen absorption.